Method for treating a gas mixture by adsorption

ABSTRACT

A method for operating a typical cyclic adsorption unit that is easily implemented in both new and existing treatment plants, wherein fluctuations in the stream compositions due to the adsorption and regeneration phase transitions of the cycles are minimized.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an adsorption process for treatment ofa gas mixture comprising at least one main constituent to be producedand impurities to be separated from said mixture, especially for theproduction of carbon monoxide with streams having predeterminedhydrogen/carbon monoxide ratios.

2. Related Art

Throughout the text, the gas pressures indicated are in bar absolute.

Such a treatment process is widely used to separate “noble” constituentsto be produced, that are contained in the gas mixture, from undesirableconstituents, generally denoted by the term “impurities”.

The typical process is cyclic and involves at least two adsorbers, of atleast two adsorption units having respectively several adsorbersoperating in common, which follow in an offset manner the same operatingcycle. This cycle conventionally comprises an adsorption phase, duringwhich the corresponding adsorber is subjected to the gas mixture andadsorbs the impurities thereof, and a regeneration phase, during whichthe adsorber is subjected to a regeneration gas and is desorbed of theimpurities that it had previously adsorbed.

Depending on whether or not the regeneration phase is accompanied byheating of the regeneration gas, it is common practice to distinguishcycles called TSA (Temperature Swing Adsorption) cycles from cyclescalled PSA (Pressure Swing Adsorption) cycles.

It is also known that the adsorbers may be subjected to depressurizationand repressurization steps and to a step of paralleling the adsorbers,during which the total stream of treated gas is obtained both by thetreatment of a first flow of gas by at least one adsorber terminatingits adsorption phase and by the treatment of a second flow of gas to betreated by at least one other adsorber starting its adsorption phase.This paralleling is conventionally intended to prevent pressure surgesin the stream of treated gas during passage in production from oneadsorber to another, especially in order to take into account theoperating time of the valves that implement the paralleling operation.

However, adsorption treatment cycles have drawbacks during transientperiods at the start of the adsorption and regeneration phases, aspartly explained in document EP-A-00 748 765.

That document describes a carbon monoxide production plant comprising acryogenic production unit and, upstream of the latter, a treatment unitthat employs a process of the type defined above. This plant is intendedto retain the water and carbon dioxide of a gas mixture rich in carbonmonoxide and in hydrogen coming from a hydrocarbon steam reforming unit.Fixing the carbon monoxide by the adsorbent of the adsorber that isstarting its adsorption phase causes an appreciable reduction in thecarbon monoxide content of the stream output by this adsorber, togetherwith fluctuations in the flow rate of this stream. The solution proposedin EP-A-0 748 765 consists in interposing, between the adsorptiontreatment unit and the cryogenic carbon monoxide production unit, a tankfilled with an adsorbent having an affinity for carbon monoxide.

This solution proves to be particularly expensive in terms ofinvestment, is not very modular and attains only to the transient periodwhen each adsorber returns to production, whereas similar transientphenomena occur at the start of the regeneration phase of each adsorber,the stream leaving the adsorbers exhibiting large fluctuations incontent and in flow rate.

SUMMARY OF THE INVENTION

The object of the invention is to propose a process of the type definedabove, that is easily implemented in the treatment plants of the priorart and that makes it possible to obviate stream perturbations due tothe absorption and regeneration phase transitions of the cycles of knownprocesses.

For this purpose, the subject of the invention is a process of theaforementioned type, in which N adsorbers are used, where N is greaterthan or equal to 2, each following in an offset manner the same cycle ofperiod T, during which there are in succession an adsorption phase and aregeneration phase using a regeneration gas, and in that each adsorberat the start of the phase and/or at the start of the use of theregeneration gas is subjected to only a portion of the nominal flow ofthe gas mixture to be treated, or alternatively of the nominal flow ofthe regeneration gas, until said adsorber is substantially saturatedwith, or alternatively substantially discharged of, at least one of themain constituents to be produced, while maintaining at least one otheradsorber in adsorption phase.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more clearly understood on reading the descriptionthat follows, given solely by way of example and with reference to theappended drawings, in which:

FIG. 1 is a schematic view of a carbon monoxide production plantaccording to the invention combined with a pure hydrogen productionunit;

FIG. 2 is a diagram illustrating the operating cycle of the adsorbers ofthe plant shown in FIG. 1;

FIGS. 3 and 4 are schematic views of the plant shown in FIG. 1 for timeintervals indicated by the numerals I and VI on the cycle shown in FIG.2; and

FIG. 5 is a schematic view of a plant for producing a stream with apredetermined hydrogen/carbon monoxide ratio according to the invention,combined with a pure hydrogen production unit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Thus, one subject of the invention is a first adsorption process for thetreatment of a gas mixture comprising at least one main constituent tobe produced and impurities to be separated from said mixture, in which Nadsorbers are used, where N is greater than or equal to 2, eachfollowing in an offset manner the same cycle of period T, during whichthere are in succession an adsorption phase and a regeneration phase,and each adsorber (11A) at the start of the adsorption phase (step I orstep IV) is subjected to only a portion of the nominal flow of the gasmixture to be treated, until said adsorber is substantially saturatedwith at least one of the main constituents to be produced, whilemaintaining at least one other adsorber (11B) in adsorption phase.

According to other features of this process:

-   -   in order to form the treated gas mixture, the stream coming from        the adsorber subjected to said portion is mixed with the stream        coming from said at least one other adsorber in adsorption        phase;    -   the duration of the adsorption phase of each adsorber is between        T/N inclusive and 2T/N noninclusive;    -   the adsorption treatment of the gas mixture is carried out—for        most of the time—by a single adsorber in adsorption phase; and    -   after said adsorber has been subjected at the start of the        adsorption phase to said portion of the nominal flow of the gas        mixture to be treated, said adsorber is subjected to a        paralleling step, during which the flow of treated gas is        obtained half by said adsorber and half by said at least one        other adsorber in adsorption phase.

Another subject of the invention is a second adsorption process for thetreatment of a gas mixture comprising at least one main constituent tobe produced and impurities to be separated from said mixture,characterized in that N adsorbers are used, where N is greater than orequal to two, each following in an offset manner the same cycle ofperiod T, during which there are in succession an adsorption phase and aregeneration phase using a regeneration gas, and in that each adsorberis subjected at the start of use of the regeneration gas to only aportion of said nominal flow of the regeneration gas, until saidabsorber is substantially discharged of at least one of the mainconstituents to be produced.

According to other features of this second process:

-   -   in order to form a discharged gas stream, the stream coming from        the adsorber subjected to said portion is mixed with the rest of        the nominal flow of the regeneration gas;    -   the stream coming from the adsorber subjected to said portion        and the rest of the nominal flow of the regeneration gas are        mixed together directly;    -   the stream coming from the adsorber subjected to said portion is        mixed with the stream coming from another adsorber that        terminates its regeneration phase and that is subjected to at        least a portion of the rest of the nominal flow of regeneration        gas;    -   the regeneration phase of each adsorber comprises a step of        depressurizing and a step of repressurizing said adsorber; and    -   the phase of regenerating each adsorber comprises a step of        heating the regeneration gas.

Shown in FIG. 1 is a carbon monoxide production plant 1 connectedupstream, via a line 2, to a hydrogen production unit 4.

The plant 1 comprises, upstream, an adsorption treatment unit 11suitable for removing most of the impurities, especially water andcarbon dioxide, that are contained in a gas mixture fed by a feed line12 and compressed to a pressure of between 15 and 45 bar. This gasmixture is compressed, for example, to below 15.5 bar and has a nominalflow rate, namely the total flow rate in the line 12, of between a fewhundred and several tens of thousands of Sm³/h. This gas mixtureincludes, as main constituents, hydrogen and carbon monoxide, at 73.5and 21.6 mol % respectively, and possibly secondary constituents, suchas nitrogen and methane, for example with respective contents of 1.1 and3.8 mol %, and it also contains, as impurities, between 10 and 200 molarppm (parts per million) of carbon dioxide, and also water, generally tosaturation.

The unit 11 comprises two adsorbers 11A, 11B placed alternately in linein order to purify the gas mixture by adsorption. Each adsorber containsan adsorbent placed either in the form of a single bed, formed from azeolite or from activated alumina optionally doped in order to increaseits carbon dioxide stopping capacity, or in the form of a plurality ofbeds formed respectively from activated alumina or from silica gel inorder essentially to stop water, and of a zeolite (for example of the A,X or LSX type) in order essentially to stop carbon dioxide. Theadsorbent may also consist of mixtures of adsorbents or of compositeadsorbents.

The treatment unit 11 also includes valves and connecting pipes whichare not shown in FIG. 1, but the arrangement of which will become moreclearly apparent during the description of the operation of this unit.

The plant 1 comprises, connected via a line 13 downstream of thetreatment unit 11, a cryogenic separation unit 14 that includes asubstantially pure carbon monoxide production line 15 and a line 16outputting a stream with a high hydrogen content. For the composition ofthe gas mixture indicated above, the stream in the line 16 may contain97.4 mol % hydrogen, 0.3 mol % nitrogen, 0.3 mol % carbon monoxide and 2mol % methane, at about 14.5 bar. Since this separation unit 14 is knownper se, it will not be explained in detail further.

The line 16 is connected to the treatment unit 11 in order to allowregeneration of the adsorber 11A, 11B that is not in the productionline, the stream having a high hydrogen content in line 16 being used,at least partly, as gas for regenerating the adsorbent of this adsorber.The total flow rate of the line 16 forms, for the example shown, thenominal flow rate of the regeneration gas.

The regeneration gas output by the adsorption treatment unit is conveyedby the line 2 to the hydrogen production unit 4, known per se. This unit4 may, for example, comprise six adsorbers operating cyclically andsuitable for producing a substantially pure hydrogen stream.

The process employed by the adsorption treatment unit 11 is obtained byrepeating the cycle illustrated in FIG. 2. Each of the two adsorbers11A, 11B follows the cycle shown in FIG. 2, with a time shift relativeto the other adsorber corresponding to a time interval equal tosubstantially one half of the period T of the cycle.

In FIG. 2, in which the times t are plotted on the x-axis and theabsolute pressures P are plotted on the y-axis, the lines headed byarrows indicate the movements and destinations of the gas currents, andalso the direction of circulation in the adsorbers 11A and 11B,respectively. When an arrow parallel to the y-axis points upwards(toward the top of the diagram), the current is said to be co-current inthe adsorber, if the arrow pointing upward lies below the lineindicating the pressure in the adsorber, the current enters the adsorberat the inlet end of the adsorber, if the arrow pointing upward liesabove the line indicating the pressure, the current leaves the adsorbervia the outlet end of the adsorber, the inlet and outlet ends beingthose for the gas to be treated and for the gas withdrawn in theproduction phase, respectively. When an arrow parallel to the y-axispoints downward (toward the bottom of the diagram), the current is saidto be countercurrent in the adsorber; if the arrow pointing downwardlies below the line indicating the pressure of the adsorber, the currentleaves the adsorber via the inlet end of the adsorber if the arrowpointing downward lies above the line indicating the pressure, thecurrent enters the adsorber via the outlet end of the adsorber, theinlet and outlet ends again being those for the gas to be treated andfor the gas withdrawn in the production phase.

The cycle shown in FIG. 2 comprises eight successive steps, denoted by Ito VIII, which will be described in succession by considering, forexample, that the absorber 11A starts its adsorption phase at time t0=0.The period T of the cycle is, for example, equal to 960 minutes for anadsorption pressure P_(ads) of about 15.5 bar.

During step I, from t0 to t1=35 minutes, the adsorbers 11A and 11B arein adsorption phase as shown in FIG. 3, the adsorber 11A receiving only5% of the flow of the gas mixture in the line 12, via a valve 111 forregulating the flow that passes through it, and the adsorber 11Breceiving the remaining 95% of the nominal flow, via a regulating valve112.

During this step, the freshly regenerated adsorber 11A stops, inaddition to the impurities (water and carbon dioxide), the carbonmonoxide contained in the gas mixture owing to the chemical affinity ofits adsorbent with carbon monoxide. Thus, the purified stream comingfrom the adsorber 11A, that passes through an open valve 113, isvirtually free of carbon monoxide. For the composition of the gasmixture indicated above, the hydrogen content of this stream leaving theadsorber 11A may reach more than 90 mol %. Concomitantly, the adsorbentof the adsorber 11B, which was saturated with carbon monoxide prior tostep I, adsorbs only the impurities from the 95% of the gas mixture thatare sent to it and produces a purified stream via an open valve 114. Thestreams from the valves 113 and 114 mix in the connection line 13 sothat the carbon monoxide and hydrogen contents of this mixture are verysimilar to their nominal values, that is to say close, for example, tothe content of the stream in this line 13 during the step that precedesstep I, the stream coming from the adsorber 11A, with a low flow rateand depleted in carbon monoxide, being diluted in the stream coming fromthe adsorber 11B.

This step I is complete when most, if not all, of the adsorbent of theadsorber 11A is saturated with carbon monoxide.

During step II, from t1 to t2=10 minutes, wherein the total elapsed timefrom t0 to t2=45 minutes, the adsorbers 11A and 11B remain in adsorptionphase, but are subjected to about 50% of the nominal flow of the gasmixture to be purified, respectively, the valves 111 and 112 beingoperated accordingly. This step II is akin to a paralleling operationwith a symmetrical distribution of the feed gas mixture, as mentioned inthe preamble of the application. This paralleling advantageously allowsthermal adjustment of the purified stream in the line 13, the streamleaving the freshly regenerated adsorber 11A having a tendency to behotter than that of the adsorber 11B at the end of the adsorption phase.

During step III, from t2 to t3=T/2=435 minutes, wherein the totalelapsed time from t0 to t3=480 minutes, only the adsorber 11A is inadsorption phase, the valve 111 being completely open, and the adsorber11B switches to the regeneration phase, the valves 112 and 114 beingcompletely closed. Thus, during most of the operating time of thetreatment unit 11 (in this case, for more than 90% of this operatingtime), the gas mixture is treated by a single adsorber 11A.

During step IV, from t3 to t4=35 minutes, wherein the total elapsed timefrom t0 to t4=515 minutes, the adsorber 11A is at the end of theadsorption phase and the adsorber 11B switches to the adsorption phase,the adsorbers 11A and 11B being subjected to 95% and to 5% of thenominal flow of the gas mixture, respectively, by means of thecorresponding adjustment of the valves 111 and 112, and by opening thevalve 114. Step IV is therefore similar to step I, the function of theadsorbers 11A and 11B being reversed.

In the same way, step V, the interval between t4 and t5=10 minutes,wherein the total elapsed time from t0 to t5=525 minutes, is similar tostep II, the function of the adsorbers 11A and 11B being reversed.

During step VI, from t5 to t6=105 minutes, wherein the total elapsedtime from t0 to t6=630 minutes, the adsorber 11A switches to theregeneration phase, the gas mixture being purified completely by theadsorber 11B, as shown in FIG. 4. The adsorber 11A is connected upstreamto the line 2 for connection to the hydrogen production unit 4, via anexpansion valve 115. The pressure in the adsorber 11A then swings fromthe pressure P_(ads) to a lower, elution pressure, denoted P_(elu), thevalue of which depends on the type of process employed in the cryogenicseparation unit 14. This elution pressure will, for example, be 1 to 2bar below the adsorption pressure P_(ads). It may also be ofsubstantially lower value, for example around 3 bar absolute.

Concomitantly with this depressurization, or once the latter has beencompleted, the adsorber 11A is subjected to the hydrogen-rich stream(regeneration gas) in the line 16, via a valve 116 for regulating theflow rate flowing in it.

This valve 116 is operated so that only 10% of the flow of theregeneration stream coming from the line 16 is sent countercurrentlyinto the adsorber 11A, the remaining 90% of the nominal flow beingconveyed directly to the connecting line 2 via a branch line 117provided with a regulating valve 118.

During application of the regeneration gas of this step VI, theadsorbent of the adsorber 11A that starts its regeneration is saturatedwith impurities (water and carbon dioxide) and with carbon monoxide. Thefirst moments of regeneration are accompanied by a strong desorption ofthe carbon monoxide, the carbon monoxide content of the stream comingfrom the adsorber 11A possibly reaching more than ten times that of theregeneration stream in line 16. Applied as such to the unit 4,especially if the latter operates by adsorption, this sudden and intenseblast of carbon monoxide would result in considerable operatingperturbations that would lead to a loss of hydrogen yield and/or tocontamination of the production by the unit 4. On the other hand, bymixing the stream coming from the adsorber 11A that has a high carbonmonoxide content with the regeneration stream of the branch line 117, inrespective proportions of 10 and 90%, the carbon monoxide content of thestream in the connecting line 2 remains at a value compatible with theoperating tolerances of the production unit 4.

This step VI continues until the adsorbent of the adsorber 11A iscompletely discharged of most of the carbon monoxide.

Advantageously, this step may continue so as to damp out the thermaleffects of the start of regeneration. This is because, since the streamcoming from the adsorber 11A tends to be cooler than the feed standardfor the unit 4, mixing it with the hotter stream, for example 20° C., inthe branch line 117 makes it possible to smooth out the temperatures ofthe stream in the connection line 2.

During step VII, from t6 to t7=160 minutes, wherein the total elapsedtime from t0 to t7=790 minutes, the elution of the adsorbent of theadsorber 11A continues by means of all of the regeneration streamconveyed by the line 16, the valve 116 being completely open and thevalve 118 being closed. Advantageously, the regeneration gas is heatedby a heater 119.

During step VIII, from t7 to t8=T=170 minutes, wherein the total elapsedtime from t0 to t8=960 minutes, the elution of the adsorbent of theadsorber 11B terminates with all of the unheated regeneration stream,then the valve 116 is closed in order to allow the adsorber to berepressurized. Step VIII is completed when the pressure of the adsorber11A reaches the value P_(ads).

The process according to the invention thus makes it possible to greatlylimit the perturbations in carbon monoxide contents both of the streamof treated gas coming from the treatment unit 11 when an adsorber startsits adsorption phase (steps I and IV), and of the stream of waste gasoutput by this treatment unit when an adsorber starts its phase of usingthe regeneration gas (step VI).

This process is easy to implement in a plant according to the prior art,that it is necessary to equip with regulating valves, such as the valves111, 112, 116 and 118, and with at least one branch line such as theline 117.

Of course, although based on the same idea of diluting the stream comingfrom the adsorber that has just started its adsorption phase or startedto be subjected to the regeneration gas with the stream coming from theadsorber terminating its regeneration phase, or alternatively with theregeneration gas directly, the implementation of steps I and IV and thatof step VI are independent, the example of the process according to theinvention described above advantageously combining both these.

During steps I or IV, the percentage of the flow of gas mixture to besent to the adsorber that is starting its adsorption phase is notlimited to 5% of the flow of the feed line 12, as in the exampledeveloped above. This percentage is generally strictly less than 50% ofthe flow in the line 12, advantageously less than one third of the flowin the line 12, preferably between 5 and 20% of the flow in the line 12.

Similarly, during step VI, the percentage of the flow of regenerationgas to be sent to the adsorber that is starting to be subjected to theregeneration gas is not limited to 10% of the flow in the discharge line16, as in the example described above. This percentage is generallystrictly less than 50% of the flow in the line 16, advantageously lessthan one third of the flow in the line 16, preferably between 5 and 20%of the flow in the line 16.

As a variant of the process and independently of the value of thepercentage of the abovementioned flows, the duration of step I or ofstep IV may be predetermined so that it is greater than about 1% of theduration of the adsorption phase of an adsorber (that is to say theperiod extending from step I to step V in the cycle shown in FIG. 2),advantageously greater than about 5% of the duration of this adsorptionphase, preferably between 10 and 20% of the duration of this adsorptionphase.

Similarly, the duration of step VI may be predetermined so that it isgreater than about 1% of the duration of the regeneration phase of anadsorber (that is to say the duration extending from step VI to VIII),advantageously greater than about 5% of the duration of thisregeneration phase, preferably between 10 and 20% of the duration ofthis regeneration phase.

Moreover, although described above with adsorbers 11A and 11B suitablefor retaining, as impurities, water and carbon dioxide, the processaccording to the invention applies to treatment units with an adsorbentsuitable for preferably fixing only water.

In addition, although described with a treatment unit 11 having only twoadsorbers, the process according to the invention applies to unitscomprising a larger number of adsorbers, operating individually or ingroups, for example operating in pairs. Thus, the term “adsorber” mustbe understood as meaning either an adsorber with its own operation, or agroup of adsorbers operating in common.

In the case of a treatment unit comprising more than two adsorbers, eachrespectively operating individually, for example three adsorbers thatfollow the same cycle with an offset substantially equal to one third ofthe cycle period, the process according to the invention proves to beparticularly advantageous when the adsorption treatment is carriedout—for most of the time—by a single adsorber in adsorption phase (asduring step III of the cycle shown in FIG. 2). More generally, for atreatment unit comprising N adsorbers, where N is greater than or equalto 2, which follow a cycle of period T, the process according to theinvention proves to be advantageous when the duration of the adsorptionphase of each adsorber is between T/N inclusive and 2T/N noninclusive.

Again in the case of a unit comprising more than two adsorbers, and oncondition that, over a given time interval of the cycle, at least twoadsorbers are in regeneration phase, the stream coming from the adsorberthat is starting to be subjected to a portion of the flow of theregeneration gas may be mixed with the rest of this flow, eitherdirectly as described above, or after the rest of this flow has beensent to another adsorber that is terminating its regeneration phase.This is because, as described during step VII and at the start of stepVIII of the cycle shown in FIG. 2, the stream coming from an adsorberthat is in regeneration for a certain time no longer has sufficientperturbations of its contents and of its flow rate. Thus, this steadystream may be used to dilute the stream coming from an adsorber that isstarting to be subjected to the regeneration gas.

As shown in FIG. 5, the cryogenic separation unit may, as a variant, bereplaced with a permeation unit 20 suitable for producing a permeatewith a predetermined hydrogen/carbon monoxide ratio, while forming anon-permeate sent to the treatment unit 11 in the same way as the streamin the line 16 for the plant 1 shown in FIG. 1. The plant 21 thus formedmakes it possible to produce a stream with a hydrogen/carbon monoxideratio that is particularly stable over time, the process according tothe invention ensuring that the unit 20 is fed via the line 13 correctlyin terms of flow rate stability, hydrogen and carbon monoxide contentsand temperature.

As a variant (not shown) of the process according to the invention, thestream coming from the adsorber that has just started its adsorptionphase or that has just been subjected to the regeneration gas may be, atleast partly, sent to a waste network to be reutilized, for example ascombustion gas (fuel gas), especially if a loss of hydrogen and/orcarbon monoxide yield by the downstream unit 14 or 20 is acceptable.

It will be understood that many additional changes in the details,materials, steps and arrangement of parts, which have been hereindescribed in order to explain the nature of the invention, may be madeby those skilled in the art within the principle and scope of theinvention as expressed in the appended claims. Thus, the presentinvention is not intended to be limited to the specific embodiments inthe examples given above.

1. An adsorption process for the treatment of a gaseous mixturecomprising at least one main constituent to be produced and impuritiesto be separated from said mixture comprising the steps of: supplyingsaid gaseous mixture to N adsorbers, wherein said N is at least two;operating each adsorber in succession through adsorption andregeneration phases in an offset manner of the same cycle of period T;providing to a first adsorber as it is starting its adsorption phase aportion of said gaseous mixture until said first adsorber issubstantially saturated; and providing to at least one other adsorber,as it is terminating its adsorption phase, the remainder of said gaseousmixture.
 2. The process according to claim 1, wherein the treatedportion from said first adsorber and the treated remainder from said atleast one other adsorber are mixed to form the treated gaseous mixture.3. The process according to claim 2, wherein a part of the treatedportion of said gaseous mixture from said first adsorber is directed toa waste network.
 4. The process according to claim 1, wherein theduration of the adsorption phase of said first adsorber and said atleast one other adsorber is between T/N inclusive and 2T/N noninclusive.5. The process according to claim 1, wherein the treatment of saidgaseous mixture is substantially provided by a single adsorberterminating its adsorption phase.
 6. The process according to claim 1,wherein after said first adsorber has been subjected at the start of theadsorption phase to said portion of said gaseous mixture to be treated,further comprises: a paralleling step, wherein about half of saidtreated gaseous mixture is obtained from said first adsorber startingits adsorption phase and half of said treated gaseous mixture isobtained from said at least one other adsorber.
 7. The process accordingto claim 1, wherein said portion of gaseous mixture to said firstadsorber comprises less than about half of the total gaseous mixture tobe treated.
 8. The process according to claim 7, wherein said portioncomprises less than about one-third.
 9. The process according to claim8, wherein said portion comprises from about 5% to about 20%.
 10. Theprocess according to claim 1, wherein the amount of time wherein saidportion of said gaseous mixture to said first adsorber comprises anamount of time greater than about 1% of the total adsorption phaseperiod.
 11. The process according to claim 10, wherein said amount oftime is greater than about 5%.
 12. The process according to claim 11,wherein said amount of time is from about 10% to about 20%.
 13. Theprocess according to claim 1, wherein said gaseous mixture compriseshydrogen and carbon monoxide as its main constituents.
 14. The processaccording to claim 13, wherein said gaseous mixture further compriseswater and carbon dioxide as its impurities.
 15. The process according toclaim 14, wherein said treated gaseous mixture is cryogenicallyseparated into a substantially pure carbon monoxide stream and ahydrogen-rich stream.
 16. The process according to claim 14, whereinsaid treated gaseous mixture is separated by permeation into a streamwith a predetermined ratio of hydrogen to carbon monoxide and into ahydrogen-rich stream.
 17. An adsorption process for the treatment of agaseous mixture consistency comprising at least one main constituent tobe produced and impurities to be separated from said mixture comprisingthe steps of: supplying said gaseous mixture to N adsorbers, whereinsaid N is at least two; operating each adsorber in succession throughadsorption and regeneration phases during the regeneration phase, in anoffset manner of the same cycle of period T; and providing to a firstadsorber as it is starting its regeneration phase, a portion of theregeneration gas until said first adsorber is substantially dischargedof said at least one main constituent to be produced.
 18. The processaccording to claim 17, wherein said portion of regeneration gas fromsaid first adsorber and the remaining regeneration gas from the at leastone other adsorber are combined to form the discharged gas stream. 19.The process according to claim 18, wherein said portion and saidremaining regeneration gas are directly mixed together.
 20. The processaccording to claim 18, wherein said portion from said first adsorber ismixed with another portion of regeneration gas coming from said at leastone other adsorber terminating its regeneration phase.
 21. The processaccording to claim 18, wherein a part of the treated portion of saidgaseous mixture from said first adsorber is directed to a waste network.22. The process according to claim 17, wherein said regeneration phaseof each adsorber comprises the steps of: depressurizing said adsorber;and repressurizing said adsorber.
 23. The process according to claim 17,wherein said regeneration phase of each adsorber further comprisesheating said regeneration gas.
 24. The process according to claim 17,wherein said portion of the regeneration gas to said first adsorbercomprises less than about half of total regeneration gas flow.
 25. Theprocess according to claim 24, wherein said portion comprises less thanabout one-third.
 26. The process according to claim 25, wherein saidportion comprises from about 5% to about 20%.
 27. The process accordingto claim 17, wherein the amount of time wherein said portion ofregenerating gas to said first adsorber comprises an amount of timegreater than about 1% of the regeneration phase period.
 28. The processaccording to claim 27, wherein said amount of time is greater than about5%.
 29. The process according to claim 28, wherein said amount of timeis from about 10% to about 20%.